Metallic components which are used at high temperatures must in many fields of engineering be protected from the oxidizing, corrosive and/or degrading attack of the operating atmosphere by means of layer systems. In the field of gas turbines, i.e. aircraft engines and stationary industrial gas turbines, it has for many years been standard practice to protect the high-temperature components from degradation by the application of layers of the MCrAl (M=Fe, Co, Ni) or β-NiAl type.
The MCrAl layers are usually applied to the metallic high-temperature component by means of vacuum or air plasma spraying.
The β-NiAl layers are applied using alitizing processes.
Depending on the particular component and the operating conditions, these types of protective layers are used both purely to protect against oxidation and corrosion and also in the form of bonding layers for ceramic thermal barrier coatings, for example based on zirconium oxide.
In both the abovementioned applications, the protective action of the layer systems is based on an aluminum oxide layer which forms on these layers at the high operating temperatures. To achieve a low growth rate and bonding of the aluminum oxide layer, the MCrAl and β-NiAl layers usually contain small quantities of elements which have an affinity for oxygen, in particular yttrium.
The aluminum content in the layer system must be sufficiently high to ensure the formation of aluminum oxide even during long-term use. In the case of β-NiAl layers, it is customary for the aluminum content to be approximately 25-30% by weight, whereas the MCrAl layers contain approximately 8-14% by weight of aluminum. The MCrAl layers have the advantage over the β-NiAl layers of being less brittle and, moreover, more resistant to corrosion in sulfur-containing operating gases.
It has been found on the basis of laboratory tests and operating experience that the long-term properties and the protective function of the MCrAl layers are crucially determined by the bonding of the thermally grown oxide (TGO) layer based on aluminum oxide which is formed on the surfaces at the high operating temperatures. This is true not only when the MCrAl layers are used to protect a metallic component against oxidation and corrosion but also in particular if they are used as a bonding layer for ceramic thermal barrier coatings.
The problems with the bonding of the TGO are based predominantly on the fact that thermally induced stresses, which are attributable to the differences in the coefficient of thermal expansion between the MCrAl layer and the TGO based on aluminum oxide, occur in and near the TGO during cooling of the MCrAl-coated component. If growth-induced cracks are formed in the vicinity of the interface between TGO and MCrAl during long-term use, the thermally induced stresses will lead to flaking of the TGO.
If the MCrAl material is used purely as a layer to protect a metallic component against oxidation and corrosion, the regular flaking of the TGO and the subsequent formation of a new oxide layer leads to accelerated consumption of the element aluminum which forms the covering layer and therefore shortens the service life of the MCrAl layer.
If the MCrAl layer is used as a bonding layer for ceramic thermal barrier coatings, flaking of the TGO will immediately give rise to flaking and therefore catastrophic failure of the thermal barrier coating.
Hitherto, it has been attempted to achieve a layer structure with a good toothed engagement with the ceramic by platinizing over the MCrAl layer with low aluminum contents of approximately 8% by weight. However, this means applying a further layer.
U.S. Pat. No. 5,741,556 shows an MCrAl layer with yttrium in which nitrogen is used as an inert gas during production of the layer.
U.S. Pat. No. 5,981,091 shows an MCrAl layer which may contain hafnium, yttrium, carbon and nitrogen. In this thermal barrier coating system, however, a platinum-enriched layer is applied to the MCrAl layer.
U.S. Pat. No. 4,774,149 discloses an MCrAl layer with hafnium, yttrium and a nitrogen content in the powder which, however, is undesirable and is to be reduced to a minimum.
U.S. Pat. No. 5,780,171 discloses an MCrAl layer comprising hafnium and yttrium, nitrogen being used as carrier gas during production of the layer.
U.S. Pat. No. 5,652,028 discloses an MCrAl layer of composition NiCoCrAl.
WO 99/23270 discloses an MCrAl layer to which lanthanum and hafnium are added.
GB 2 243 161 A discloses an MCrAl layer with additions of zirconium, silicon, tantalum, hafnium, yttrium, scandium or lanthanum.
U.S. Pat. No. 5,141,821 discloses an MCrAl layer with included particles of carbides in order to improve the abrasion properties of the layer. Furthermore, the MCrAl layer may contain zirconium, hafnium and tantalum.